Not nitpicking, I'm aware of the comment HCW made regarding the call. But your comment was -
He hasn't said that. Yes the call would be important if they can identify the other party but they have also made appeals for british holiday makers to review their photographs for example, which could be equally important. And they are appealing for people who know the interiors of his homes. Without knowing what other evidence they have, we don't really know what else could clinch the case for them and what is really crucial. Even if they do place CB via the call he could still claim to have been in the area for another purpose, it's not a clear indicator of guilt and HCW even says they would still need more evidence (in addition to placing CB on the phone).
Plus we don't know what the original eyewitnesses like JJ and TS have already told police, do they already confirm it is CB they saw? Is the phonecall just another further nail in the coffin?
I'm just saying the phonecall may not be the be all and end all of their case. Anything that places CB at the crime scene would be fine, the call is just one angle they are looking at. And ultimately if there is a video or something of MM and they can find that location, the phonecall and placing CB in the area may become a moot point if something more significant is unearthed.
The 'crucial' thing would be indentifying a direct link with CB to MM - forensic evidence, a body, the place she was taken, a murder weapon. Something along those lines.